1. Field of the Invention
The present invention relates to a fuel supply apparatus for and a pressure control method of an internal combustion engine, and particularly relates to a technique of controlling pressure in a fuel tank.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 05-272417 discloses an apparatus for determining that a hole or leak has occurred when an inside of the fuel tank is pressurized by a pump and a pressure in the fuel tank cannot be increased to a predetermined pressure.
Japanese Unexamined Patent Publication No. 05-180098 also discloses an apparatus for diagnosing whether a leak is present or absent based on a reduction in a fuel-tank internal pressure in response to an intake negative pressure of an internal combustion engine which negative pressure is introduced into a fuel tank to prevail thereinside.
Furthermore, Japanese Unexamined Patent Publication No. 2004-162685 discloses an apparatus including a pump for drawing out air in a treatment path of a fuel vapor, through a canister and for diagnosing whether a leak of fuel vapor is present or absent based on a reduction in pressure in the treatment path of fuel vapor when the pressure is reduced by the pump.
However, even if execution of pressurization or pressure reduction is stopped to open the fuel tank to the atmosphere through the canister when the leak diagnosis is completed, it may take a long time for an internal pressure of the fuel tank to become a pressure in a steady state (i.e., at around the atmospheric pressure) in some cases, depending on a tank shape or a condition of the canister.
When the leak diagnosis is carried out by pressurizing the inside of the fuel tank, if the internal pressure of the fuel tank is kept high even after the diagnosis is completed, and if there is a leak, a leakage of the fuel vapor from the leak becomes large.
When the leak diagnosis is carried out by reducing the pressure inside the fuel tank, since the fuel is likely to be vaporized when the pressure in the fuel tank is low, if the pressure-reduced state continues for a long time after the diagnosis is completed, a large quantity of fuel vapor may be generated. The large quantity of fuel vapor generation will enrich an air-fuel ratio of the engine. Moreover, if a large quantity of fuel vapor is generated, the fuel vapor may be easily permitted to leak or flow toward the outside of a vehicle.